INTERNET-DRAFT Editor: Kurt D. Zeilenga
Intended Category: Standards Track OpenLDAP Foundation
Expires in six months 10 Febuary 2003
Updates: RFC 2595
The Plain SASL Mechanism
<draft-ietf-sasl-plain-00.txt>
Status of Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
This document is intended to be, after appropriate review and
revision, submitted to the RFC Editor as a Standards Track document.
Distribution of this memo is unlimited. Technical discussion of this
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Copyright 2003, The Internet Society. All Rights Reserved.
Please see the Copyright section near the end of this document for
more information.
Abstract
This document defines a simple clear-text user/password Simple
Authentication and Security Layer (SASL) mechanism called the PLAIN
mechanism. The PLAIN mechanism intended to be used, in combination
with data confidentiality services provided by a lower layer, in
protocols which lack a simple password authentication command.
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Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [Keywords].
1. Background and Intended Usage
Clear-text passwords are simple, interoperate with almost all existing
operating system authentication databases, and are useful for a smooth
transition to a more secure password-based authentication mechanism.
The drawback is that they are unacceptable for use over an unencrypted
network connection.
This document defines the PLAIN Simple Authentication and Security
Layer ([SASL]) mechanism for use in protocols with no clear-text login
command (e.g., ACAP).
The name associated with this mechanism is "PLAIN".
The PLAIN SASL mechanism does not provide a security layer. This
mechanism MUST NOT be used without adequate security protection as the
mechanism affords no integrity nor confidentiality protection itself.
The PLAIN SASL mechanism MUST NOT be advertised unless a strong
encryption layer, such as provided by Transport Layer Security
([TLS]), is active.
This document updates RFC 2595, replacing Section 6. Changes since
RFC 2595 are detailed in Appendix A.
2. PLAIN SASL mechanism
The mechanism consists of a single message from the client to the
server. The client sends the authorization identity (identity to
login as), followed by a NUL character, followed by the authentication
identity (identity whose password will be used), followed by a NUL
character, followed by the clear-text password. The client leaves
the authorization identity empty if wishes the server to derive the
authorization identity from the provided the authentication identity.
The formal grammar for the client message using Augmented BNF [ABNF]
follows.
message = [authzid] NUL authcid NUL passwd
authcid = 1*SAFE ; MUST accept up to 255 octets
authzid = 1*SAFE ; MUST accept up to 255 octets
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passwd = 1*SAFE ; MUST accept up to 255 octets
NUL = %x00
SAFE = UTF1 / UTF2 / UTF3 / UTF4 / UTF5 / UTF6 / UTF7
;; any UTF-8 encoded Unicode character except NUL
UTF1 = %x01-7F
UTF2 = %xC0-DF 1(UTF0)
UTF3 = %xE0-EF 2(UTF0)
UTF4 = %xF0-F7 3(UTF0)
UTF5 = %xF8-FB 4(UTF0)
UTF6 = %xFC-FD 5(UTF0)
UTF0 = %x80-BF
The authorization identity (authzid), authentication identity
(authcid) and password (passwd) SHALL be transferred as [UTF-8]
encoded strings of [Unicode] characters. As NUL (U+0000) is used as a
deliminator, the NUL (U+0000) MUST NOT appear in authzid, authcid, or
passwd productions.
The form of the authzid production is specific to the
application-level protocol's SASL profile [SASL]. The authcid and
passwd productions are form-free. Use of non-visible characters or
characters which a user may be unable to enter on some keyboards is
discouraged.
Servers MUST be capable of accepting authzid, authcid, and passwd
productions up to and including 255 octets. It is noted that the
UTF-8 encoding of a Unicode character may be as long as 6 octets.
Upon receipt of the message, the server will verify presented
authentication identity (authcid) and password (passwd) with the
system authentication database and that the authentication credentials
permit the client to login as the (presented or derived) authorization
identity. If both steps succeed, the user is authenticated.
The presented authentication identity and password strings are not be
compared directly with stored strings. The server SHALL first prepare
authentication identity strings and password strings using the
"username" and "password" profiles [SASLPrep], respectively, of the
"stringprep" algorithm [StringPrep]. If preparation fails or results
in an empty string, verification SHALL fail. If the server stores
only the hash of expected string, that string MUST be prepared before
generation of the hash.
The verification function (using hashed password) can be written (in
psuedo-code):
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boolean Verify( string authzid, string authcid, string passwd ) {
string pAuthcid = prepareUserName( authcid ); # prepare authcid
string pPasswd = preparePassword( passwd ); # prepare passwd
if(pAuthcid == NULL || pPasswd == NULL) {
return false; # preparation failed
}
if(pAuthcid == "" || pPasswd == "") {
return false; # empty prepared string
}
storedHash = fetchPasswordHash(pAuthcid);
if (storedHash == NULL || storedHash == "") {
return false; # error or unknown authcid
}
if (!compare( storedHash, Hash(pPassword))) {
return false; # incorrect password
}
if (authzid == "") {
authzid = deriveAuthzid( pAuthcid );
if (authzid == NULL || authzid == "") {
return false; # could not derive authzid
}
}
if (!Authorize(pAuthcid, authzid)) {
return false; # not authorized
}
return true;
}
The server MAY also use the password to initialize any new
authentication database, such as one suitable for [CRAM-MD5] or
[DIGEST-MD5].
4. Example
Here is an example of how this might be used to initialize a CRAM-MD5
authentication database for ACAP. "C:" and "S:" indicate lines sent
by the client and server respectively.
S: * ACAP (SASL "CRAM-MD5") (STARTTLS)
C: a001 AUTHENTICATE "CRAM-MD5"
S: + "<1896.697170952@postoffice.reston.mci.net>"
C: "tim b913a602c7eda7a495b4e6e7334d3890"
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S: a001 NO (TRANSITION-NEEDED)
"Please change your password, or use TLS to login"
C: a002 STARTTLS
S: a002 OK "Begin TLS negotiation now"
<TLS negotiation, further commands are under TLS layer>
S: * ACAP (SASL "CRAM-MD5" "PLAIN" "EXTERNAL")
C: a003 AUTHENTICATE "PLAIN" {21+}
C: <NUL>tim<NUL>tanstaaftanstaaf
S: a003 OK CRAM-MD5 password initialized
In this example, <NUL> represents a single NUL (U+0000) character.
5. Security Considerations
The PLAIN mechanism relies on the TLS encryption layer for security.
When used without TLS, it is vulnerable to a common network
eavesdropping attack. Therefore PLAIN MUST NOT be advertised or used
unless a suitable TLS encryption layer is active or backwards
compatibility dictates otherwise.
When the PLAIN mechanism is used, the server gains the ability to
impersonate the user to all services with the same password regardless
of any encryption provided by TLS or other network privacy mechanisms.
While many other authentication mechanisms have similar weaknesses,
stronger SASL mechanisms such as the Kerberos-based GSSAPI mechanism
address this issue. Clients are encouraged to have an operational
mode where all mechanisms which are likely to reveal the user's
password to the server are disabled. It is RECOMMENDED that this mode
be the default.
General SASL security considerations apply to this mechanism.
"stringprep" and Unicode security considerations also apply.
6. IANA Considerations
It is requested that the SASL Mechanism registry [IANA-SASL] entry for
the PLAIN mechanism be updated to reflect that this document now
provides its technical specification.
To: iana@iana.org
Subject: Updated Registration of SASL mechanism PLAIN
SASL mechanism name: PLAIN
Security considerations: See RFC XXXX.
Published specification (optional, recommended): RFC XXXX
Person & email address to contact for further information:
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Kurt Zeilenga <kurt@openldap.org>
Chris Neuman <chris.newman@innosoft.com>
Intended usage: COMMON
Author/Change controller: IESG <iesg@ietf.org>
Note: Updates existing entry for PLAIN
7. Acknowledgement
This document is a revision of RFC 2595 by Chris Newman.
8. Normative References
[ABNF] D. Crocker, P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[Keywords] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14 (also RFC 2119), March 1997.
[SASL] J. Myers, "Simple Authentication and Security Layer
(SASL)", RFC 2222bis (a work in progress).
[SASLPrep] TBD, "SASL String Preparation Profiles", (a work to
come).
[Stringprep] P. Hoffman, M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0", defined by: The Unicode Standard, Version 3.0
(Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),
as amended by the Unicode Standard Annex #28: Unicode 3.2
(http://www.unicode.org/reports/tr28/tr28-3.html).
[UTF-8] F. Yergeau, "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[TLS] T. Dierks, C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
9. Informative References
[CRAM-MD5] J. Klensin, R. Catoe, and P. Krumviede, "IMAP/POP
AUTHorize Extension for Simple Challenge/Response", RFC
2195, September 1997.
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[DIGEST-MD5] P. Leach, C. Newman, "Using Digest Authentication as a
SASL Mechanism", RFC 2831, May 2000.
[IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
MECHANISMS", http://www.iana.org/assignments/sasl-
mechanisms.
10. Editor's Address
Kurt Zeilenga
OpenLDAP Foundation
Email: kurt@OpenLDAP.org
Appendix A. Changes since RFC 2595
This appendix is non-normative.
This document replaces Section 6 of RFC 2595.
The specification details how the server is to compare client-provided
character strings with stored character strings.
The ABNF grammar was updated. In particular, the grammar now allows
LINE FEED (U+000A) and CARRIAGE RETURN (U+000D) characters in the
authzid, authcid, passwd productions. However, whether these control
characters may be used depends on the string preparation rules
applicable to the production. For authcid, control characters are
prohibited. For passwd, control characters are allowed. For authzid,
one must consult the application-level SASL profile.
Full Copyright Statement
Copyright 2003, The Internet Society. All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
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copyrights defined in the Internet Standards process must be followed,
or as required to translate it into languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE AUTHORS, THE INTERNET SOCIETY, AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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